These studies are directed towards developing information to describe the enhanced cell killing by X-rays found with certain but not all anti-cancer agents. Of particular interest is the variation in the sensitivity to such radiosynergism found with tumor lines from different sources. Two assays for the repair of radiation damage in DNA are studied; X-ray induced repair replication and the rejoining of X-ray induced DNA strand breaks. The results of these two molecular assays are compared to cell survival studies performed after various drug treatments in culture. The data to date indicates that radiosensitization is most usually based on the interaction of primary drug induced damage but in some cases radiosensitization (of oxygenated cells) can be shown to be based on the inhibition of cellular (enzymatic) repair processes. Variations in the capacity of different cell lines to repair X-ray damage to DNA has also been found.